To switch between them, as well as other alternate appearances, unlocked from, for instance, the Collectors Edition, and future DLC, use the Cycle Appearance button that appears at the bottom of the squad selection screen, as seen here:
Here's a terrible quality screenshot of everyone but EDI.
And here's the alt appearance for EDI since it's not in that screenshot.
As for the bonuses provided, here's most of the list, I could use some help with Kaidan.
Garrus: +25% Weapon Damage
EDI: +25% Shields
Liara: +25% Power Recharge Speed
James: +25% Weapon Damage
Ashley: +25% Weapon Damage
Kaidan: +25% Shields
Tali: +25% Weapon Damage